Bioacoustics of Ischnocnema crassa

Natural History. Ischnocnema crassa is a nocturnal leaf-litter frog inhabitant of the forest interior. Two individuals were found along transects inside the forest and seven individuals were found across the 507 plots of 5 m x 5 m sampled in Santa Teresa. Nine individuals were inside the forest at 50 m (N=1), 150 m (N=1), 200 m (N=4), and 300 m (N=3) from the forest edge. No individuals of I. crassa were in the 100 plots sampled at the forest edge and in the 84 plots inside the matrix of abandoned pasture, coffee plantation, or Eucalyptus plantation, and thus the population density estimate excluded these 184 plots. By assuming the probability of detection = 1 across the used habitats (323 plots inside the forest), the estimated population density of I. crassa is 0.11 individual/ 100m 2 in Santa Teresa. Males called during rainy nights under the leaf litter inside the forest. In a laboratory aquarium, a pair of I. crassa displayed acoustic and tactile courtship signals. During courtship on 16 July 2019 , a male MNRJ 93652 (SVL 1.95 mm) vocalized by inflating the single, median and subgular vocal sac. The amplexus of I. crassa is axillary ( Fig. 4F ), similar to that reported for I. verrucosa ( Lirio et al . 2018 ) and I. lactea ( Silva-Soares et al . 2018 ) . For 10 min in amplexus, the male performed head movements by rubbing the chin from left-to-right direction on the female’s dorsum. The adult female MNRJ 93653 (SVL 22.2 mm) collected on 29 June 2019 had 13 mature eggs (xÞ= 3.0 ± 0.2, 2.4–3.1 mm) and two immature eggs. Observations of calling males, amplexus, and eggs in oviducts during May to August suggest I. crassa breeds during colder and drier season. On the field, I. crassa displayed seven antipredator mechanisms and nine variations: camouflage (variation: background matching; Fig. 7C ), immobility ( Fig. 7A and 7C ), interrupt calling, aposematism (variation: hidden), posture (variations: body elevation [ Fig. 7B ], body inflation [ Figs 7A–B, and 7D ], death feigning, mouth gape), escape (variations: hide and jump away [ Fig. 7C ]), cloacal discharge, and aggression (variation: kick). Ischnocnema crassa camouflages on the leaf litter due to the dorsal coloration similar to a decomposing leaf ( Fig. 7A and 7C ). During our approach, a male and a female adults jumped and hid on the leaf-litter. During capture attempts, adults jumped and remained motionless on the leaf litter.Adults have contrasting coloration on venter that ranges from dark brown (male) to dark gray (female) distinctively areolate with small golden irregular blotches. During handling and manipulation, adults inflated the body, elevated the body and death feigned showing the hidden aposematism, opened the mouth, kicked with posterior limbs, and released cloacal discharge. No individual released skin secretion. Five species co-occurred with Ischnocnema crassa within the 5 m X 5 m plots: one individual was found with Euparkerella tridactyla Izecksohn, 1988 (N=1) and Ischnocnema aff. parva sp. 1 (N=1). Another individual was found with Adelophryne glandulata Lourenço-de-Moraes, Ferreira, Fouquet & Bastos, 2014 (N=1) and Haddadus binotatus (N= 1). The other two individuals were found with A. glandulata (N= 1), H. binotatus (N= 1), I. aff. parva sp. 1 (N= 4), and I. verrucosa (N= 1). We did not observe any interspecific interaction.